1. Field of the Invention
The present invention relates to an electrically operated power steering apparatus on a motor vehicle which applies the drive torque of an electric motor as an assistive steering force directly to a steering system to reduce the manual steering force that the driver of the motor vehicle is required to exert to steer the motor vehicle.
2. Description of the Prior Art
Recent years have seen widespread use of electrically operated power steering apparatus which have an electric motor for applying an assistive steering torque to a steering system and control the drive force of the electric motor depending on the manual steering torque applied to the steering system for thereby reducing the manual steering force that the driver of a motor vehicle exerts to steer the motor vehicle.
It has been customary in the electrically operated power steering apparatus to control the output torque of the electric motor by supplying the electric motor with a target value in the form of a voltage or a current that is determined depending on the manual steering torque which is detected by a manual steering torque detector.
In the conventional electrically operated power steering apparatus, no consideration is given to the moment of inertia and viscosity resistance of the rotatable member of the electric motor. Therefore, if the moment of inertia and the viscosity resistance are of a large value, then large inertia and viscosity torques are imposed in a direction opposite to the torque generated by the electric motor. When the electric motor is started, it may produce a response delay in producing an assistive torque due to the inertial resistance thereof. When a countertorque from the tires is applied at the time no assistive torque is generated, since the rotational member of the electric motor is coupled to the rack, the moment of inertia of the electric motor is applied to the steering wheel. Therefore, no appropriate assistive torque can be achieved simply by supplying the electric motor with a target value in the form of a voltage or a current that is determined depending on the manual steering torque which is detected by a manual steering torque detector. As a result, the feel of steering action has been relatively poor with the conventional electrically operated power steering apparatus.